The present disclosure relates to the field of absorption spectroscopy. More specifically, it provides a means for analysis of chemically complex samples using gas chromatography (GC) separation with vacuum ultra-violet (VUV) spectroscopy detection.
There are a wide variety of complex chemical samples for which analysis may be desired. A particular type of known sample analysis is the analysis of hydrocarbon content in petroleum-based fuels such as unleaded gasoline, jet fuel, and diesel fuel. However other complex liquid and gas samples are known to be subject to analysis, including but not limited to food products, fragrances, etc. For the purposes of this specification, the term “complex” refers to samples containing many constituents, often hundreds or more constituents, the constituents generally being distinct molecular species that can largely be separated by a chromatographic separation process.
One example of a “complex” sample for which analysis is often desired is petroleum-based liquid fuels. Petroleum fuels can consist of several hundred to several thousand distinct components. Most of the molecular components of petroleum products fall into one of a handful of distinct categories. For example, saturated hydrocarbons are carbon and hydrogen containing compounds where all bonds between carbon atoms are single bonds. Saturated hydrocarbons can be thought of as being saturated with hydrogen atoms, as they have the largest number of hydrogen atoms possible given the number of carbon atoms present and their conformational structure. Replacing one or more of the single carbon-carbon bonds with double bonds reduces the degree of saturation. Saturated hydrocarbons are often further classified into linear chain paraffins, branched chain isoparaffins, and cyclic naphthenes. Unsaturated hydrocarbons where one or more double carbon-carbon bonds exist are known as olefins, diolefins, or polyolefins. Aromatic compounds are ring structures that contain some degree of aromaticity. The simplest is benzene, consisting of a six-membered carbon ring, with six of its electrons being completely delocalized. Benzene is sometimes also represented as a ring structure with three alternating double carbon-carbon bonds. Various other aromatic compounds consist of a benzene base with some of the hydrogen atoms replaced by various other functional groups. For example, toluene is a benzene derivative with a methyl group replacing one of the hydrogen atoms. Aromatic compounds that contain a single aromatic ring are often known as monocyclic aromatic hydrocarbons, or simply mono-aromatic compounds. Multiple ring aromatic compounds also exist, and are often known as polycyclic aromatic hydrocarbons (PAHs). The simplest PAH with fused aromatic rings is naphthalene. The simplest PAH with two separated aromatic rings is biphenyl. As in the case of mono-aromatic compounds, a large variety of compounds are possible having one of these bases along with various functional groups replacing the terminating hydrogen atoms. Molecules that have PAH cores with substituents in place of some of the hydrogen atoms are not technically PAHs, but they are still commonly referred to as such, and this practice will also be used in the current specification. For the purposes of this specification, the term “poly-aromatic” is also used as a generic term for molecules having multiple (two or more) aromatic rings.
Some specific molecular components are of particular importance in fuel analysis, such as iso-octane or benzene. Individual oxygenates such as ethanol can also be important. However, a variety of properties of petroleum-based fuels depend mainly on the relative amounts of bulk classes of compounds. For example, measurement of bulk concentration of the various hydrocarbon classes is important for fuel production control. The total amount of aromatics is often regulated due to environmental concerns. Olefin content is also highly regulated in some markets, especially in Europe.
One type of bulk classification of gasoline-range fuels is characterization of relative mass or weight content of each of the five hydrocarbon groups of paraffins, isoparaffins, olefins, naphthenes, and aromatics (for the purposes of this specification, the terms mass percent and weight percent will be used interchangeably). This type of analysis is often known by its acronym PIONA. Combinations of hydrocarbon classes may be used to obtain other types of bulk classifications. For example, the three classes of saturated hydrocarbons included in a PIONA analysis are sometimes combined to quantify the total amount of saturated hydrocarbons in a sample. Such analysis may further determine bulk mono-aromatic and bulk poly-aromatic content of a fuel sample. Classification of hydrocarbon classes can be accompanied by characterization of specific molecular species in a sample, such as quantifying individual oxygenates or members of the BTEX (benzene, toluene, ethylbenzene, xylenes) aromatics complex.
Gas chromatography is an analytical method for separating complex mixtures into their constituent components, which are then quantified and/or identified using a variety of detectors. A typical mode of operation for a GC experiment is to vaporize a small amount (typically 0.1-1 μL) of liquid sample and forcing it via pressurized carrier gas onto the head of an analytical column. The carrier gas is also known as the mobile phase, and is usually an inert gas such as nitrogen or helium. In a common type of GC, known as capillary GC, the analytical column is a long, 15-100 m fused silica capillary whose inner walls are coated with a film coating known as a stationary phase. A pressure differential is maintained between head and detector ends of the column, causing the carrier gas to continuously flow through the column. The analytical column is contained in an oven. The starting temperature of the oven is usually room temperature or lower, and a significant portion of the vaporized sample will re-condense at the head of the analytical column. The oven temperature is then gradually increased, liberating condensed species into the mobile phase in a manner consistent with the species' boiling points. The stationary phase interacts with the various molecular constituents traversing the column, retaining them differently depending on the species' chemical makeup. Thus the separation process consists of a gross separation depending on constituent boiling point combined with a stationary phase separation driven by the chemistry of interaction between constituent molecules and stationary phase molecules. The result is that the individual constituents making up the original sample elute from the column at different times. For a thorough separation process, each molecular constituent elutes at a unique elution time. This is not always possible, however, and it is possible that multiple distinct species will coelute at a given elution time. The term “analyte” is used often throughout this disclosure to describe a molecular species that elutes from the analytical column during a GC experiment.
One of a variety of detectors is placed at the end of the analytical column, generating a signal when analyte molecules exit the column. The detector response signal as a function of time is known as a chromatogram. The distribution for most compounds is driven by random interaction with the column phase as they traverse the column and tends toward Gaussian. The chromatogram of a well-separated mixture consists of a series of approximately Gaussian shaped “peaks”, each representing the elution of one of the analyte constituents.
For the simplest type of chromatogram consisting of a plot of response versus time, the elution time is used to identify the compounds. An analyte's elution time is also known as its retention time, and is specific to a given GC method and analytical column. The size of the response, either the height or area of the peaks, is used to quantify the amount of the constituents. In most cases, a relative measurement is done, and the relative amounts of various constituents and solvent are used to determine constituent concentration in the original liquid sample.
The most common gas chromatographic detector for hydrocarbon analysis is the flame ionization detector. Flame ionization detection (FID) shows good sensitivity and linearity to carbon-containing compounds. In addition, FID is generally cheap and robust. The main disadvantages of FID are insensitivity to some compounds of interest, such as carbon dioxide and water, but especially the inherently one-dimensional response. The FID has no ability to distinguish between different compounds, and in particular has no ability to deal with coelution, i.e., when multiple compounds elute simultaneously from the analytical column. This means that complete separation has to be achieved in order to quantify individual compounds in a substance. GC-FID analyte identification is only possible by using the elution order of compounds from a sample. For a substance containing many individual constituents, like petroleum-based fuels, very accurate knowledge of elution order is required, and this elution order must be strictly maintained from run to run. A method describing a GC-FID methodology for analysis of refined gasoline samples is given in the American Society for Testing and Materials (ASTM) method D6730. The type of analysis done in this method is often referred to as detailed hydrocarbon analysis (DHA), and focuses on identifying and quantifying as many specific compounds as possible in a gasoline mixture. The requirement for complete separation results in an extremely long run time of up to three hours (“fast” DHA methods claim to accomplish this in about 1½ hours). The requirement for strictly establishing and maintaining elution order results in a very complicated setup procedure. Since analytical columns degrade with use, the setup procedure has to be done periodically when the analytical columns are replaced.
Coelution can be tolerated if bulk classification is desired, but even in this case complete separation by analyte class is still necessary. In the case of fuel analysis, this results in very complicated separations involving multiple columns, traps, and switching valves. An example of such an apparatus, also known as a reformulyzer, is described in ASTM D6839. The reformulyzer is difficult to set up and maintain, and run times tend to be long, typically consisting of an hour or more.
Other GC detectors add a level of identification capability independent of the elution time. Mass spectrometry fragments eluting analytes into characteristic mass spectra, in the form of a response versus mass-to-charge ratio. Mass spectrometry is a mature technology and large libraries exist enabling identification for a large number of unknown compounds in a sample. As a GC detection technique, mass spectrometry has several disadvantages. It is inherently destructive. It is insensitive to several types of compounds and only very elaborate and expensive variations can distinguish between structural isomers or stereoisomers of the same compound. Also, mass spectra can be very difficult to interpret and lack intuitive class-based features that result in easy molecular class identification.
Infra-red (IR) spectra can be used to distinguish compounds from each other, but IR wavelengths are completely insensitive to ground state to excited state electronic transitions, tending to be sensitive to much lower cross-section vibrational transitions. As such, IR absorbance is much less sensitive to a given amount of molecule than VUV absorbance spectra. In addition, quantification using IR spectroscopy is challenging due to difficulties associated with maintaining the high resolution requirements, both in terms of tool matching and variation in individual instruments over time.
In spite of being mature, prolific technologies, neither mass spectrometry nor IR spectrometry has become a serious contender for bulk classification analysis of complex samples in general. In the particular case of complex fuel mixtures, ASTM methods like D6730 and D6839 remain the dominant methods of analysis.
It would be desirable to have an improved method for analyzing complex samples.